1. Field of the Invention
This invention relates to a bearing assembly and more particularly to a new and novel bearing assembly for use in supporting a water lubricated propeller shaft as in large naval ships.
2. Description of the Prior Art
Bearing assemblies with elastomeric bearing elements are particularly suited for this purpose because of their excellent ability to withstand the effects of corrosive fluids and to abrasion resulting from particulars of foreign matter carried in suspension in the sea water in which the shaft and bearing assembly operates. Such bearing assemblies with their elastomeric bearing elements have been made and are still being made with outer non-corrosive support or shell with a plurality of circumferentially evenly spaced elastomeric staves therein.
Lubricated bearings commenced in the 1880""s with the discovery of considerable fluid pressure in the oil-film of bearings, from which oil-lubricated bearings were developed. In the 1890""s tilting-pad oil-lubricated metal bearings were invented and later tilting-pad journal bearings were developed. In tilting-pad bearings, the pads physically move rather than deflect because of the very high modules or stiffness of the metal, to give the right approach angles required to develop very low hydrodynamic self pumping lubrication.
The term xe2x80x9celastohydrodynamicxe2x80x9d evolved to cover those dynamic cases (like gear teeth) where the bearing surfaces were rapidly moving in and out of contact yet were apparently hydrodynamically lubricated, if only for periodic, very short time intervals. It was later learned that the low friction of certain oil-lubricated plastic bearings was due to a fluid-trapping pocket being formed in the plastic.
Early water-lubricated rubber bearings were ineffective because the bearings did not develop hydrodynamic lubrication. Water-lubricated bearings in the 1960""s were the category of medium wear devices, the same category as automotive brake linings. This was because the rubber was too thick, the surface was too rough and the side edges were rounded. Thin rubber bearings have different wear and friction action from harder nonmetallic materials used in bearings. When a radial load is initially applied to a thin rubber bearing, the deflection is elastic and disappears if the load is removed. After a short period of time under load, the deformation becomes permanent and remains after load removal because of the compression set present in every rubber compound. This action is called plasto-elastohydrodynamic lubrication. This type of lubrication is enhanced by the special elastic-creep properties of some rubber compounds. It was found that thin nitrile rubber bearings for rotating shafts or journals partially immersed in water developed a continuous, unbroken film of water resulting from hydrodynamic lubrication. Tests of rotating journals thin rubber bearings formed a plastic-elastohydrodynamic pocket in the rubber. It was determined that the pressure developed by the hydrodynamic pumping process deformed the rubber, pushing it out towards the sides and ends of the bearing contact patch, forming a non-contact region or pocket in the center of the contact patch i.e. plasto-elastohydrodynamic (xe2x80x9cPEHxe2x80x9d) lubrication. It was further found that bearing test sample patches made of polymeric materials harder than rubber initially deflected less than the samples made of thin rubber, but eventually developed deep grooves and wore away after partially completing the test twenty eight day procedure.
There have been developed brass backed rubber staves since the 1920""s, and plastic back rubber staves since 1976. The prior art further includes rubber in compression impinging on slippery surfaces such as Teflon, ultra high molecular weight poletheylene (UHMWPE) and other materials. See U.S. Pat. Nos. 3,743,306, 4,568,056, 4,626,112 and 4,737,688.
An object of the invention is to provide an improved bearing assembly for a rotating shaft.
Another object is to provide an improved bearing element for a bearing assembly for use with a rotating shaft.
Still a further object is to provide an improved bearing assembly using lubricated trapping pockets for reducing the friction for a shaft rotating in the assembly.
It is an additional object to provide a non-metal bearing stave for supporting or assisting in the support of a shaft rotating in water or other fluid, such as a propeller shaft for a ship or boat.
Yet still a further object is the provision of an improved bearing stave for use in a low friction, low wear bearing.
Another object is to provide a bearing assembly for the rotating shaft of a ship or boat, the assembly having one or more staves with reduced stave length to shaft diameter ratios over prior bearing assemblies with staves.
Another object is to provide a bearing assembly for rotating shafts having three staves which have improved performance over prior three staved bearing assemblies.
Yet an additional object is to provide a staved bearing assembly for rotating shafts of ships or boats having a reduced number of components from prior staved bearing assemblies for ships or boats.
Another object is to provide an improved bearing elements for a bearing assembly for rotating shafts, having lower breakaway friction than prior bearing elements.
Yet another object is to the provision of an improved bearing element for a bearing assembly for rotating shafts, having lower low speed friction than prior bearing elements. Another provision is an improved bearing element for a bearing assembly for rotating shafts having zero drum wear.
It is another object of the invention to provide an improved bearing element for a shaft and bearing assembly capable of operating for a longer system wear life than did prior such bearing elements under higher contact pressure. A further object is to provide a method for making a bearing assembly with staves having discontinuous surfaces for supporting rotating shafts, the staves made from slippery polymer alloy xe2x80x9cSPAxe2x80x9d).
A further object is to provide a method for making a bearing stave made from SPA.
A general object is to provide a bearing stave and a bearing assembly for rotating shafts which is of improved performance, efficiency and efficiency from the prior art, and which can produced using currently available economic materials.
The present invention is directed to a novel bearing assembly utilizing an outer shell and a plurality of circumferentially spaced bearing elements in contact with the shaft, the bearing elements having transverse grooves or a hydrodynamic pattern provided therein to thereby reduce the bearing friction torque and the wear thus enhancing the performance of the bearing.
The present invention further includes a bearing assembly with one or more elements such as staves for supporting the rotating shaft of a ship or boat, where the staves have discontinuous surfaces such as lands formed by a series of transverse grooves extending across the width of the staves or at protuberances or contact points distributed over the stave surfaces, and are made from SPA.
The discontinuous surfaces of the bearing elements multiply contact the rotating shaft, and develop individual conforming water lubricated polymer alloy miniature bearings. During operation, water lubricant pockets develop as a result of PEH effect, to essentially lift the rotating shaft from the bearing element(s) to reduce the friction and wear of the bearing elements. The invention is particularly useful for high pressure (load), on the order of from 250 to 3000 kPa (35 psi to 435 psi). The length-to-diameter (L/D) ratio is reduced, and has been found to enable successful L/D ratios of from 2.0 all the way down to 0.85 or lower, rather than the conventional LID ratio value of from 4 to 7.
An unexpected aspect to the present invention is that whereas one would have expected the friction to increase as the contact area of the rotating shaft by the bearing decreased (pressure increased), in fact just the opposite happened. As the contact area of the bearing decreased with increased load by virtue of the discontinuous surfaces of the bearing, the friction and wear significantly decreased. The discovery of this phenomena is an essential aspect of the present invention.